This invention generally relates to circuit materials, methods of manufacture of the circuit materials, and articles formed therefrom, including circuits and multilayer circuits.
As used herein, a circuit material is an article used in the manufacture of circuits and multilayer circuits, and includes circuit subassemblies, non-clad or declad dielectric layers, single or double clad dielectric layers, prepregs, build-up materials, bond plies, resin-coated conductive layers, cover films, and the like. A circuit laminate is a type of circuit subassembly that has a conductive layer, e.g., copper, fixedly attached to a dielectric substrate layer. Double clad laminates have two conductive layers, one on each side of the dielectric layer. Patterning a conductive layer of a laminate, for example by etching, provides a circuit. Multilayer circuits comprise a plurality of conductive layers, at least one of which contains a conductive wiring pattern. Typically, multilayer circuits are formed by laminating two or more materials in proper alignment together, at least one of which contains a circuit layer, using bond plies, while applying heat or pressure.
Dielectric layers include “prepregs,” typically comprising a fibrous reinforcement that is impregnated with a resin system that is partially cured (“B-staged”). A prepreg can also be referred to as a bonding sheet. Thus, a prepreg can be B-staged to obtain an intermediate stage in the reaction of the thermosetting resin used in the material, after being held at an elevated temperature for a sufficient time to volatilize formulation solvent. After B-staging, the prepreg can be stored for an extended period of time prior to fully curing the material during the manufacture of a circuit laminate or other circuit subassembly. In one type of construction, multilayer laminates can comprise two or more plies of prepregs between metal layers.
In the art of circuit laminates, preferred dielectric materials for use in the manufacture of substrates, bond plies, and the like are characterized by lower dissipation factors (Df), especially for high performance circuit applications, for example, operating at high frequency or at high data transfer rates. Df is a measure of loss-rate of energy of an electrical oscillation in a dissipative system. Electrical potential energy is dissipated to some extent in all dielectric materials, usually in the form of heat, and Df can vary depending on the dielectric material and the frequency of the electrical signals. Df can be especially relevant to a printed circuit board (PCB) antenna, a critical component in any transmission system or wireless communication infrastructure, for example, cellular base station antennas. For high performance applications, a dielectric constant (Dk) of less than 3.5 and a Df of less than 0.006 are desirable.
In addition to low dissipation factor, another consideration in the selection of material components for high performance electronic applications is low flammability. Low flammability can be difficult to achieve because the lower polarity polymers preferred for lowering Df can also exhibit greater flammability, resulting in the need for flame retardant additives in circuit materials. The selection and amount of flame retardant, however, may need to be limited in order to avoid adversely affecting electrical properties, thermal stability, water absorption, chemical resistance, and other properties such as peel strength.
Still other properties of interest for high performance circuit materials are high Tg, for heat resistance, and low coefficient of thermal expansion (X, Y, Z CTE) for dimensional stability and plated-through-hole (PTH) reliability.
Improving one property of a circuit material, however, can adversely affect another property of the circuit material. Thus, for example, a higher Tg material may be at the expense of a lower Df, so that obtaining a desired matrix of properties can be a challenge in developing an improved circuit material.
Moreover, not only do the electrical and physical properties of the final product need to be considered, but so do the properties of the formulation used to manufacture the final product. Thus, another consideration in the selection of a component material used in the manufacture of a high performance circuit material is its impact during the manufacture of the circuit material. In particular, there is a need to curtail or limit polymer or prepolymer flow to prevent runback during prepreg manufacture. Also, during lamination, when a prepreg is typically heated under pressure, the polymer or prepolymer melts (liquefies) and flows. The prepreg polymer or prepolymer content, pressure, and the rate of heating of the system can impact the amount of flow that occurs during lamination. There is a finite period of time during which the polymer or prepolymer remains sufficiently fluid to flow freely, after which, if curing is used, the average molecular weight increases to the point at which it becomes solidified or “gelled.” A certain amount of flow of the polymer or prepolymer is desirable in the lamination process and can impact such properties as interlaminar bond, fill and flow into patterned features of adjacent layers, bonding to copper foil and final dielectric thickness (Hf). Thus, the viscosity or flow characteristics of a prepreg, both during manufacture and during subsequent lamination, can be important for obtaining good manufacturing performance.
In order to more efficiently cure a thermosetting resin system of a circuit material during manufacture and subsequent processing, such as lamination, a crosslinking agent can be employed to react with the thermosetting resin. Common crosslinking agents are triallyl isocyanurate (TAIC) and triallyl cyanurate (TAC), typically in monomeric liquid form. Although capable of providing high cross-link density, the use of monomeric triallyl isocyanurate monomer can cause runback against the glass or other fabric reinforcement. Moreover, triallyl isocyanurate monomer can be fugitive in treater equipment, depending upon the B-staging or other conditions used to manufacture a prepreg.
U.S. Pat. No. 3,576,789 discloses poly(TAIC) in powder form and a particle size of less than five microns, preferably less than one micron, which powder can be prepared by emulsion or suspension polymerization. The patent mentions blending such particles with polypropylene or other polyolefin above the melting point of the polyolefin. Crosslinking of other polymers is not mentioned.
U.S. Pat. No. 4,962,168 and U.S. Patent Publication 2013/0334477 disclose the polymerization of TAIC monomer in the presence of an oxaphosphorine flame retardant (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide or “DOPO”). Specifically, the patent mentions TAIC “prepolymers” of low molecular weight, although mentioning a broad range of 800 to 80,000 and noting that a molecular weight of 20,000 to 80,000 tends to give a powder. The examples in the patent, however, obtained a molecular weight ranging from 1,180 to 27,480, as measured by GPC. As background information, the patent mentions that possible uses of TAIC in general include electronic materials, synthetic resins, paints, adhesives, and other industrial materials, although. The examples in the patent, however, are directed to impregnating a laminate paper. Again, crosslinking of other polymers is not mentioned.
U.S. Pat. No. 6,254,971 discloses (Example 7) a circuit material comprising both poly(TAIC) and monomeric TAIC, wherein the poly(TAIC) is the basic resin. Other examples in the patent include resin systems based on poly(phenylene ether) or PPE, but there is no example of a mixture of PPE and poly(TAIC) or of poly(TAIC) dispersed in a matrix polymer system.
U.S. Pat. No. 6,734,259 and Patent Publication 2012/0045955 disclose prepreg circuit materials comprising a combination of PPE, TAIC, and silica, but not poly(TAIC). Finally, Patent Publication 2012/0164452 discloses poly(TAIC) in the form of microbeads (1 to 100 microns in diameter) for use in HPLC. U.S. Patent Publication 2015/0105505 discloses a reactive trimer of dibromostyrene and dibromostyrene in a varnish composition for making a prepreg.
U.S. Pat. No. 7,138,470, U.S. Patent Publication 2013/0334477, and related patents disclose the possible use of poly(triallyl isocyanurate) or the like as co-agents in the vulcanization of fluoroelastomers.
In view of the above, there remains a need for improved high performance circuit materials comprising precursor compositions, especially low polarity compositions, for use in a circuit material. Specifically, there is a need for circuit materials having an improved combination of properties, including high Tg and low Df, among other desired electrical and physical properties. Furthermore, such circuit materials, even when containing significant amounts of monomeric TAIC for crosslinking, are desirably capable of being efficiently manufactured and of providing low flammability during use.